There are many medical procedures in which probes, such as catheters, are introduced into the body of a subject or patient. In procedures such as cardiac catheterization and neurosurgery, it is often necessary for the physician or surgeon to know the location of the distal end of the probe inside the body. Although imaging methods such as fluoroscopy and ultrasound are sometimes used for this purpose, they are not always practical or desirable. For example, such systems typically require continuous imaging of the probe and patient during the procedure. In addition fluoroscopic systems are often undesirable because that they expose the patient and physician to substantial ionizing radiation.
A number of locating systems for detecting the position of a probe or a catheter tip in the body of a patient without the need for continuous imaging of the patient have been proposed. These systems include, for example, those disclosed in U.S. Pat. Nos. 5,558,091; 5,391,199; 5,443,489; and International Patent Publications WO 94/04938 and WO 96/05768, the disclosures of which are hereby incorporated herein by reference. Other electromagnetic tracking systems, not necessarily for medical applications, are described in U.S. Pat. Nos. 3,644,825, 3,868,565, 4,017,858, 4,054,881 and 4,849,692.
Systems such as those disclosed in the '091, '199, and '489 patents and in the '938 PCT application determine the disposition (i.e., position, orientation, or both) of a probe using one or more field transducers, such as a Hall effect devices, magnetoresistive devices, coils or other antennas carried on the probe. The transducers are typically located at or adjacent the distal end of the probe or at a precisely known location relative to the distal end of the probe. Such systems further utilize one or more reference field transducers disposed outside the body to provide an external frame of reference. The reference field transducers are operative to transmit or detect non-ionizing fields or field components such as magnetic field, electromagnetic radiation or acoustical energy such as ultrasonic vibration. By transmitting fields between the external reference field transducers and the probe field transducers, characteristics of the field transmissions between these devices can be determined and then used to determine the position and orientation of the probe in the external frame of reference.
As described, for example, in the aforementioned '091 patent, the frame of reference of the external field transducers can be registered with the frame of reference of imaging data such as magnetic resonance imaging data, computerized axial tomographic ("CAT") data, or conventional x-ray imaging data, and hence the position and/or orientation data derived from the system can be displayed as a representation of the probe superimposed on an image of the patient's body. The physician can use this information to guide the probe to the desired location within the patient's body, and to monitor its location and orientation during treatment or measurement of the internal body structure. This arrangement greatly enhances the ability of the physician to navigate the distal end of the probe through bodily structures and offers significant advantages over conventional methods of navigating probes within the body by feel alone. Because it does not require acquiring an optical image of the surrounding tissues for navigation purposes, it can be used with probes which are too small to accommodate optical elements. These transducer-based systems also avoid the difficulties associated with navigation of a probe by continuous imaging of the probe and patient during the procedure and avoids, for example, prolonged exposure to ionizing radiation inherent in fluoroscopic systems.
Such systems typically utilize reference field transducers or coils which are provided in a fixed, immovable array, in locations such as on the ceiling of an operating room or rigidly fixed to operating or catheterization table. In medical applications, where the system is used to track the location of a probe inside the body of a patient, the coil mounting may interfere with free access by the physician to the patient.
For example, the aforementioned '938 publication describes a catheter system which uses a plurality of non-concentric coils adjacent to the distal end of the catheter. These coils generate signals in response to externally applied magnetic fields, which allow for the computation of six location and orientation coordinates, so that the disposition of the catheter is known without the need for simultaneous imaging. Preferably, at least three such coils or radiators are arrayed in fixed locations outside the body, adjacent to the area of the body into which the catheter is introduced. For example, in cardiac catheterization, during which the patient is typically supine, three radiators are typically fixedly placed beneath the patient's thorax, in a fixed coplanar, triangular arrangement, with the centers of the coils from about 2 to 40 cm apart. For detection of the position and orientation of catheters or probes inserted into the brain, the transducers or field radiating coils should desirably be positioned adjacent to the patient's head. In neurosurgery, however, the patient if often in a seated, upright position or else face-down. Thus, a triangular frame holding the three radiators as described above cannot be comfortably and stably positioned below the head. However, positioning the frame above or beside the head will generally interfere with the surgeon's manipulation of probes and surgical tools.
It would therefore be desirable to enhance the accuracy and efficacy of probe tracking systems described above and other types of systems involving application of electromagnetic or other non-ionizing energy fields to a human body, by adjusting and optimizing the positions of the reference field transducers. Flexibility of placement of the transducers would allow custom positioning of the transducers to move them to the best possible locations to increase sensitivity of the locating system.